Only five rivers in the world carry more water than the Yarlung Zangbo, or Brahmaputra as it is known when it reaches India. Only one carries more silt. Rising at a height of 5,300 metres in the Kailash range of the Middle Himalayas – an area holy to both Hindus and Buddhists – the river flows east through Tibet for 1,625 kilometres before taking a horseshoe bend, changing its name and flowing as the Brahmaputra into north-eastern India.

There, for 918 kilometres, it is both a lifeline due to the water it carries and a scourge because of the floods it causes almost every year. It then takes a southward turn and flows into Bangladesh for 363 kilometres before it merges with the Ganges, together forming south Asia’s largest river, the Meghna, and flowing into the Bay of Bengal. This huge river, with its 25 large tributaries in Tibet and 105 in India, drains much of the eastern Himalayas.

As the world’s youngest mountain range, the Himalayas are particularly unstable – and so is the river. It has changed its course significantly at least once in the last 200 years, following a major earthquake. Smaller changes in course are common, wiping out farms and homes on one bank while depositing fertile silt on the other. Now humans are changing the course of this river: Chinese engineers have started to build the Zangmu hydroelectric power station in Lhoka prefecture, 325 kilometres from Lhasa, Tibet’s capital. The development has led to serious expressions of concern, particularly in India but also in China.

Chinese plans on the Brahmaputra are nothing new. In June 1996, the Scientific American first reported China’s intention to divert the river to its north-west territory, mostly covered by the Gobi desert. China’s dam projects have long been a source of controversy. Critics say they cause huge environmental problems and do little to control floods, while millions of people are displaced. Earlier this year, Chinese dams were accused of channelling water away from the upper reaches of the Mekong River and contributing to the waterway’s record low levels – a charge Beijing has dismissed [PDFver here] .

The Tibet Online version of the People’s Daily [PDFver here] reported that construction of the Zangmu power station started on November 12. The appearance of the report led to immediate criticism from many experts in India and one in China. Though the Indian government has not made any official statement since building started, over the past three years it has repeatedly raised the issue with the Chinese government, expressing concern that the project could disrupt water supplies downstream in India and harm ecosystems.

Now China’s foreign ministry spokesman Hong Lei has found it necessary to brief the media [PDFver here] on the subject. “In the development of cross-border water resources, China has always had a responsible attitude and places equal emphasis on development and protection,” he said, adding that China took “full consideration of the potential impact on the downstream area”.

Chinese engineers are asking why India is so worried. Li Chaoyi, chief engineer at China Huaneng Group, the project’s main contractor, told [PDFver here] news agency Xinhua: “The river will not be stopped during construction… After the project becomes operational, the river water will flow downstream through water turbines and sluices. So the water volume downstream will not be cut.”

But India is worried, particularly about one part of the Xinhua report, which said the project “can also be used for flood control and irrigation”. This would require diversion and storage of water, experts have pointed out. There will be major impact downstream if any of the 79 billion cubic metres of water that flows down the Brahmaputra into India every year is diverted or reduced. “The diversified fauna and flora there have evolved over tens of millions of years and will be damaged,” the Global Times quoted Wang Yongchen, the founder of Beijing-based Green Earth Volunteers, as saying.

Speaking on condition of anonymity, a senior official from India’s Ministry of Water Resources said: “While power generation could either be a storage project or a run-of-the-river project, the flood control feature requires storage structures. And the irrigation feature would mean water would be diverted. These features are of concern to India.”

The Zangmu scheme will be the first “mega hydroelectric power plant on the Tibetan plateau”, according to the Chinese media. There will be six 85-megawatt power-generating units, the first of which is expected to start working in 2014 and to reduce the serious power shortages that now afflict Tibet. The project, which is expected to cost nearly 7.9 billion yuan (US$1.2 billion) according to the Global Times, is a key project for Tibet in China’s 11th Five-Year Plan. According to available preliminary information, the Chinese plan to have a series of five medium-sized dams along the river in this area of Tibet.

The issue has been raised in the Indian parliament more than once. On April 22, India’s foreign minister SM Krishna told [PDFver here] the upper house of parliament: “It is a fact that when I met my Chinese counterpart recently, the question of the hydel [hydroelectric] project over Brahmaputra river being built by it in Zangmu did come up. However, the Chinese foreign minister assured me that it is a small project which will not have any impact on the river’s downstream flow into north-east India.” Pressed further, he said: “With reference to trans-border rivers, we have an expert level mechanism to address the issue. A meeting of experts from both India and China is scheduled to take place April 26 to 29 in Delhi and the issue will be discussed in it.”

The discussion evidently did not satisfy the Indian government, which appointed a group of its most senior bureaucrats – led by the cabinet secretary – to keep a watch on the project. Using data gathered by satellites, the officials alerted the Indian media about the construction in Zangmu almost as soon as it started.

One of the main reasons the Indian government is worried is that it has planned similar hydroelectric projects in its stretch of the river – plans that are under fire from environmentalists. If there is a change in the volume of water flowing into India, those plans will go awry. And the country further downstream, Bangladesh, will probably object to the Indian plans. Swelled by its tributaries while it flows through India, the Brahmaputra carries a huge 570 billion cubic metres past Guwahati, the capital of Assam province, shortly before it enters Bangladesh. It is the major source of water in northern Bangladesh, and any change in its volume is likely to affect the country adversely.

A pure run-of-the-river project may not affect the water volume, since it channels the water through the hydroelectric turbines and then releases it further down the river, but it does affect the amount of silt a river carries. Hydroelectric engineers do not want silt as it clogs up their turbines, and often find ways to get rid of it before the water enters their channel. But the nutrient-rich sediment is vital for agriculture downstream, both in India and Bangladesh. The Brahmaputra has one of the largest catchment areas in the world – about 580,000 square kilometres – and most of the people within it are farmers.

The Brahmaputra leaves the Tibetan plateau in the eastern Himalayas, which is one of the richest areas in the world in terms of biodiversity. According to WWF, at least 353 new species were discovered in the eastern Himalayas between 1998 and 2008, an average of 35 new species finds every year. Located at the crossroads of two continental plates, the eastern Himalayas supports many of the threatened Bengal tigers and is the last bastion for the greater one-horned rhinoceros. But the biodiversity of the region is already under immense pressure due to deforestation, agriculture, unsustainable fuel wood collection, overgrazing by domestic livestock, illegal poaching, mining, pollution, hydropower development and poorly planned infrastructure.

The impact of these threats is exacerbated by the region’s great vulnerability to climate change. There are 612 glaciers in the Brahmaputra basin. And these glaciers are receding due to global warming. Only 25% of the region’s original habitats remain intact and 163 species that live in the eastern Himalayas are considered globally threatened, according to WWF. Experts say any change in the Brahmaputra’s water and silt volume is likely to have a further adverse impact on this biodiversity.

Chinese officials have pointed out that the Zangmu project is similar to the Baglihar dam, built by India on the Chenab River before it flows into Pakistan. The difference is that India is bound by the Indus Water Treaty to ensure that the project does not reduce the volume of water flowing into Pakistan. India and China do not have such a treaty. Indian officials have said in the past that they have sought a similar agreement without success.

However, in 2006 the two countries agreed to establish an expert-level mechanism to discuss trans-border issues related to using the river as an economic resource and have since signed an agreement for sharing flood-related hydrological data for the Brahmaputra during monsoon season. During the period from June 1 to October 15 each year to 2012, China will provide hydrological data twice a day to India to help better manage floods. After 2012, a fresh implementation agreement will be needed.

Indian officials say their attempts to expand this cooperation beyond the peak flood season have so far not succeeded. Nervous governments and communities along the Brahmaputra will be waiting to see if their neighbourhood can find a way to work more closely together – or if the fears surrounding the Zangmu project will prove to be justified.